Mobile broadband capacity is growing fast; there is currently 70% growth in data traffic worldwide year on year and most of this growth relates to in-building use. Today's small cell and Distributed Antenna System, DAS, solutions inadequately address the broad range of buildings that mobile communications network operators may want to provide network coverage within, due to a lack of cost effective and well performing solutions that scale well to different building types. With evolving capacity needs, indoor mobile network solutions need to move from current DAS models and distributed femto/pico cells to a heterogeneous small cell model that permits efficient re-use of an operator's macro spectrum inside a building.
Small cell alternatives and active DAS solutions have emerged as contenders in the in-building mobile network space, but have thus far not been strong enough to make any significant impact outside specific use cases. The challenge for small cell products is to meet size, volume and cost targets and that has resulted in products based on System-on-a-Chip platforms (SoC), giving limited performance and functionality compared with the products typically used in a macro network. This has in turn ruled out the use of small cell in performance critical indoor use cases, where regular macro base stations with DAS are being used. The key challenges faced by DAS alternatives are their inability to scale down cost to medium and small size buildings, and that there is no possibility of cost-effectively densifying the indoor cell grid at spectrum capacity exhaustion.
Copper based links set tight constraints on the maximum distance between a radio unit, RU, and a remote radio head, RRH. In medium-large buildings, digital units, DU, and indoor RUs can be centrally co-located, with local area network, LAN, copper cabling connecting the RU to one or more RRH. For large to very large buildings, the DU can be centrally located in the building with indoor RU's distributed at floor level to reduce cabling lengths to the RRH. In situations where a radio base station, RBS, or a micro RBS is already provided on the building roof or nearby, DU baseband resources can be shared, with only the indoor RU's and RRH being installed within the building, effectively leveraging the operator's installed base. This subtending model also enables advanced long term evolution, LTE, coordination between outdoor and indoor network coverage areas. In multi-building campuses or very large venues, a centralized DU can be shared among facilities, while still supporting future expansion. All these scenarios require proximity of RU and RRH to overcome distance limitations imposed by copper links.
The use of optical fibre between RU and RRH, by radio over fibre, RoF, techniques, is one of the best ways to centralize radio functions. RoF can be done in either the analogue or the digital domain. Analogue RoF is able to reduce latency, to be fully agnostic to the carried radio signal and also to simplify RRH complexity, by terminating the digital domain in the RU. Unfortunately, analogue RoF suffers from cumulative effects of noise and device nonlinearities as well as crosstalk arising from impairments in the optical link. In addition, the analogue RoF link performance heavily depends on the current of the photoreceiver used to receive the RoF optical signal, so that the link attenuation that can be tolerated can be very low, limiting the optical power distribution by tree architecture and passive power splitters and also the number of antenna signals which can be carried by a single optical carrier.
Digital RoF can overcome these limitations and the common public radio interface, CPRI, is the reference technology for Digital RoF, as defined in the CPRI Specification v6.0 of 30 Aug. 2013. CPRI data that has to be transferred in the optical domain from a radio base station to an RRH, and vice versa, are transferred in the form of sampled IQ data. A daisy chain of CPRI links can be used to connect a DU to a series of RUs and their connected local RRHs, as illustrated in FIG. 1. Depending on the bandwidth demand of the RU, CPRI can transport several independent IQ data flows. Each IQ data flow reflects the data of one RRH for one carrier, the so-called antenna-carrier, A×C. Although CPRI performs well in macro centralized installations, it is a quite inefficient way to distribute indoor radio channels, characterized by large numbers of small-cells. CPRI requires high bandwidth, dedicated connectivity and a tight synchronization between all the antennas served by a common main digital unit, DU.